Catalysis is literally the lifeblood for many industrial/commercial processes in the world today. The most important aspect of a catalyst is that it can increase the productivity, efficiency and profitability of the overall process by enhancing the speed, activity and/or selectivity of a given reaction. Many industrial/commercial processes involve reactions that are simply too slow and/or inefficient to be economical without a catalyst present. For example, the process of converting natural gas or methane to liquid hydrocarbons (an extremely desirable process) necessarily involves several catalytic reactions.
The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is catalytically reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas intermediate is catalytically converted to higher hydrocarbon products by processes such as the Fischer-Tropsch Synthesis. For example, fuels with boiling points in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes may be produced from the synthesis gas.
Current industrial use of methane as a chemical feedstock for syngas production proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming or dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, the reaction proceeding according to Equation 1.CH4+H2O⇄CO+3H2  (1)
The catalytic partial oxidation (“CPOX”) of hydrocarbons, e.g., methane or natural gas, to syngas has also been described in the literature. In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a more preferable H2:CO ratio of 2:1, as shown in Equation 2:CH4+½O2⇄CO+2H2  (2)
The H2:CO ratio for this reaction is more useful for the downstream conversion of syngas to chemicals such as methanol or to fuels than is the H2:CO ratio from steam reforming. However, both reactions continue to be the focus of research in the world today.
As stated above, these reactions are catalytic reactions and the literature is replete with varying catalyst compositions. The catalyst compositions typically are comprised of at least one catalytically active metal, such as a Group VIII metal. Many catalyst compositions also have other promoters present. Catalytic metals are typically selected based on their activity and selectivity towards a particular reaction. Further, the catalyst compositions typically include particular support materials such as alumina, silica, titania, etc., that can also enhance the catalyst activity.
After a period of time in operation, a catalyst will become deactivated, losing its effectiveness for catalyzing the desired reaction to a degree that makes the process uneconomical at best and inoperative at worst. This process is generally known as “aging.” The more aged a particular catalyst the less efficient the catalyst is at enhancing the reaction, i.e., less activity it has. At this point, the catalyst can be either replaced or regenerated. However, replacing a catalyst typically means discarding the deactivated catalyst. Even if a fresh replacement catalyst is ready and available, a single syngas reactor will typically have to be shut down and offline for days to weeks. The time delay is due at least in part to the time required for simple cooling and heating of the reactor.
In addition, a discarded catalyst represents a loss of expensive metals. Alternatively, the user may send the catalyst back to the supplier for recovery of expensive metals, such as Rh, Pt, Pd, etc. However, the recovery process involves dissolving the multi-component catalyst and subsequent separation of the active components from the mixed solution such as a method described in co-owned and pending patent application Ser. No. 10/176,224. The chemistry is complex and costly, more importantly, it involves bulk amounts of harsh chemicals that ultimately must be discarded and the use of landfills for such disposal is problematic. For example, the environmental protection agency (EPA) “Land Ban” imposes restrictions on disposal because these harsh chemicals can release toxins into the environment. For all of these reasons, regeneration is preferred over replacement.
However, regeneration has problems as well. Like replacement, regeneration typically requires some downtime resulting in a decrease in production. In addition, regeneration may not be possible for every deactivated catalyst. Catalysts systems can become deactivated by any number of mechanisms. Some of the more common deactivating mechanisms include coking, sintering, poisoning, oxidation, and reduction. The process chiefly responsible for deactivation varies among catalyst systems. Some catalysts that have been deactivated can be regenerated and/or the deactivation reaction can be reversed. However, many regeneration processes are not economically feasible.
Sintering as a cause of deactivation has traditionally been viewed as a non-reversible phenomenon, since a sintered catalyst is particularly difficult to regenerate. In terms of synthesis gas catalysts, sintering is usually the result of the high temperatures within the catalyst bed. The syngas reactions achieve very high temperatures during operation. Temperatures within a syngas catalyst bed typically reach temperatures in excess of 1000° C. Sintering for syngas catalysts is practically unavoidable.
Chemical reaction of sintered metals and/or its compounds with some reagents to form volatile compounds has been found to be effective in redispersing sintered active metals. For example, formation of platinum oxychloride has been used to disperse platinum.
In U.S. Pat. No. 5,106,798, a procedure is disclosed for regeneration of zeolite supported noble metal, such as platinum, catalysts. One of the steps in the disclosed process is to redisperse the noble metal on the surface of the catalyst support by contacting the catalyst with halogen gas and carbon dioxide.
U.S. Pat. No, 4,925,819 discloses a method of regenerating a reforming catalyst comprising a type L zeolite containing a group VIII noble metal via an oxychlorination procedure. The catalyst is prepared by heating the catalyst in the presence of oxygen, hydrogen or other inert gas, and water. The catalyst surface is then chlorinated under similar conditions. Finally, the chlorine is removed primarily by heating under the appropriate chemical atmosphere.
U.S. Pat. No. 5,260,238, described a method of regenerating catalyst comprised of Noble metal/zeolite, in which the deactivated catalyst was first de-coked through reduction/oxidation, then contacting the deactivated catalyst with a solution containing a halogen or a galogen-containing compound and thereafter calcining the deactivated catalyst.
Chemical atmosphere also has a strong effect on rhodium dispersion on metal oxide supports. Zaki et al reported in J. Phys. Chem. 1987, 91, 1486-1493 that “successive cycles of CO adsorption and regeneration of a catalyst in H2 causes deterioration of the dispersion and progressive formation of larger Rh aggregates”.
It was also reported that rhodium carbonyl can be used as precursor to graft Rh on metal oxide support (L. Basini etal, J. Phys. Chem., 1992, 96, p9431-9441) through the following reaction:Rh4(CO)12+[M]-OH→[M]-O—RhI(CO)2+H2+COwhere M is a metallic element that is contained on a support.
Because regeneration of syngas catalysts has traditionally been so difficult, the active metals are typically dissolved and recaptured for use in new catalyst batches. However, research is continuing on the development of more efficient syngas catalyst systems and catalyst systems that can be more effectively regenerated. To date there are no known methods that are economically feasible for regenerating a sintered syngas catalyst.
Hence, there is still a great need to identify new regeneration methods, particularly methods that are quick and effective for regenerating deactivated syngas catalysts without having to dissolve the catalyst components and without significant downtime or loss of production.